Nitrogen trifluoride (NF.sub.3) gas has recently drawn attention as a dry etching agent for semiconductors and a cleaning gas for CVD apparatuses, and nitrogen trifluoride gas used for these purposes is required to be as highly pure as possible.
Nitrogen fluoride (NF.sub.3) gas may be produced by various methods, but nitrogen fluoride gas produced by any method mostly contains impurities such as nitrous oxide (N.sub.2 O), carbon dioxide (CO.sub.2), dinitrogen difluoride (N.sub.2 F.sub.2) and the like in a relatively large amount. Therefore, it is required to purify the NF.sub.3 gas so as to obtain a highly pure NF.sub.3 gas usable for the above-mentioned purposes.
As a method for purifying NF.sub.3 gas by removing these impurities, a method which comprises adsorbing and removing the impurities by means of an adsorbent such as zeolite and the like is known as one of the most efficient and simplest methods [Chem. Eng., 84,116 (1977), etc.]. However, when N.sub.2 F.sub.2 is present in the NF.sub.3 gas, the purification method by adsorption suffers the following drawbacks.
That is,
1) When N.sub.2 F.sub.2 is present, the adsorption ability of the other impurities such as CO.sub.2, N.sub.2 O and the like is extremely lowered.
2) When N.sub.2 F.sub.2 is present, NF.sub.3 is liable to be adsorbed to the adsorbent resulting in a loss of NF.sub.3 gas.
3) N.sub.2 F.sub.2 adsorbed to the adsorbent and thereby concentrated is liable to decompose and generate heat, and in an extreme case, an explosion is caused.
Therefore, when impurities in NF.sub.3 gas are to be removed by using an adsorbent such as zeolite and the like, it is necessary to remove N.sub.2 F.sub.2 in advance.
As a method for removing N.sub.2 F.sub.2 contained in NF.sub.3 gas, a method is known which comprises reacting N.sub.2 F.sub.2 with an aqueous solution of KI, HI, Na.sub.2 S, Na.sub.2 S.sub.2 O.sub.4, Na.sub.2 SO.sub.3 or the like in a reaction vessel [J. Massonne, Chem. Ing. Techn., 41, (12), 695 (1969)]. However, according to this method, it takes a relatively long time to remove N.sub.2 F.sub.2 completely and therefore the reaction vessel must be considerably large and a large amount of chemicals is also necessary.
As another known method for removing N.sub.2 F.sub.2, NF.sub.3 gas containing N.sub.2 F.sub.2 is passed through a catalyst packed bed formed by packing a reaction vessel with metal pieces or nets which has a sufficient reactivity capable of defluorinating N.sub.2 F.sub.2 such as heated stainless steel, carbon steel, copper, aluminum, zinc, lead, nickel, iron and the like, and thereby, when the NF.sub.3 gas is brought into contact with the metal pieces or net as a catalyst, the N.sub.2 F.sub.2 is decomposed on the surface of the metal pieces or net (U.S. Pat. No. 4,193,976). However, according to the present inventors' investigation, this method is liable to form metal fluorides on the surface of the metal pieces or net due to the reaction of the metal pieces with N.sub.2 F.sub.2. And the resulting metal fluorides are very fragile, are easily peeled off from the surface of the metal pieces and powdered. As a result, there is a drawback their that the powder clogs the packed bed and pipes and the like of the purification apparatus.
According to our investigation, when nickel is used as the metal pieces, the fluoride film is formed on the surface of the nickel pieces, but after the surfaces of the nickel pieces have become covered with the fluoride film, there is no exposed nickel metal surface remaining for the reaction with N.sub.2 F.sub.2 so that the nickel pieces naturally have lost the catalytic activity. Therefore, it is necessary to stop the operation periodically and substitute new nickel pieces for the old ones so as to newly pack the catalyst bed. Therefore, the operation is very complicated and moreover, nickel is expensive, and therefore, in combination, the operation cost is disadvantageously high.
In addition, according to the present inventors' investigation, when the heating temperature of the packed bed composed of metal pieces is elevated so as to enhance the rate of the removal of N.sub.2 F.sub.2, the main component NF.sub.3 also reacts with the metal pieces at temperatures of 200.degree. C. or higher to decompose and the yield of NF.sub.3 is decreased to a great extent corresponding to the degree of decomposition.
The present inventors have diligently researched for a method for removing N.sub.2 F.sub.2 contained in NF.sub.3 gas and surprisingly found that N.sub.2 F.sub.2 is efficiently decomposed into nitrogen (N2) gas and fluorine (F2) gas by only heating the NF.sub.3 gas containing N.sub.2 F.sub.2 to specified temperatures without bringing the gas into contact with any metal surface. When the above-mentioned heating is effected in a specified vessel the wall surface of which is covered with a passive film, there is the advantage that the main component, NF.sub.3, of the gas is not substantially decomposed even if the gas is heated to a temperature of 200.degree. C. or higher.
In addition, the present inventors have also found that when a solid fluoride is packed in the above-mentioned vessel, the N.sub.2 F.sub.2 is more efficiently subjected to decomposition and therefore, only N.sub.2 F.sub.2 in NF.sub.3 gas can be effectively removed.
The present invention has been completed based on the above-mentioned new knowledge found by the present inventors.